Electronic photoflash guns have been extensively used for some years to provide extra illumination for photography in low ambient light conditions. A requirement of control circuits associated with photoflash guns has been to time accurately the firing of the photoflash tube(s) and also to provide a definite cut-off or quench of the photoflash tube when sufficient light has been generated. This latter cut-off can be either fixed to provide one or more discrete levels of generated light (whereupon exposure settings within the camera may need to be varied to compensate) or may be automatically provided when a quantity of light sufficient for a predetermined exposure setting of the camera has been generated by the tube.
Prior control circuits for photoflash guns as will hereinafter be described operate on the principle of charging and discharging capacitors in order to switch thyristors in order to initially fire the flash gun and to cut off or quench the flash gun. However, operation by charging and discharging capacitors can be slow, particularly where fast recycling times are required, for example in motor driven cameras where it may be necessary to take sequential exposures very rapidly and a photoflash gun must be able to recycle in a very short time between exposures.